A synchronization process in a communication system is divided into a cellular network and a private network. The cellular network absolutely determines the time when a transmission packet should be transmitted to operate a system, and the private network permits arbitrary timing. The cellular network is a commercial network that should transmit data efficiently with bands allocated to multiple users and should save frequency/time resources as much as possible. However, in the case of a wireless local area network (LAN), a wireless personal area network (PAN), or Ethernet, in which importance of resources is not great, transmission and reception timing is not restricted. That is, a receiving end receives a packet at a start point of a packet determined by a transmitting end and transmits a response at transmission timing thereof. In other words, in a synchronization process of the transmitting and receiving ends, it is necessary to search only the start point of the packet transmitted by the transmitting end. However, in the cellular network, all terminals can transmit a packet simultaneously and a packet should be transmitted at a specific time point of a network which is to be accessed by the terminals in order to access the network while reducing interference between the terminals. To this end, a synchronization process is required. A base station transmits a reference time for synchronization through a synchronization channel and terminals confirm a reference time of a system through the synchronization channel. A random access channel (RACH) is a region in which a terminal disregards ambiguity caused by a physical distance thereof and can transmit a signal received by a base station. Through this physical channel, the base station receives a signal from the terminal and permits searched terminals to access a network.
A channel for a base station to receive a random access signal is defined as an RACH in the 3rd generation partnership project (3GPP) long term evolution (LTE) and as a ranging channel in IEEE 802.16. A signal transmitted to such a channel uses Zadoff-Chu constant amplitude zero autocorrelation (CAZAC) in the 3GPP LTE and uses a pseudo-noise (PN) sequence in the IEEE 802.16. When a terminal accesses an RACH, the terminal selects one of three predetermined sequences and transmits the selected sequence at a determined time. A base station searches the three sequences. If the base station determines that a received signal is present, the base station broadcasts a response to a corresponding sequence ID.
A signal transmitted by a transmitting end is distorted by a channel between the transmitting end and a receiving end and then is transmitted to the receiving end. At a time point when this signal is detected, signals transmitted by multiple mobile stations are simultaneously discovered. In this case, performance is determined based on a signal structure used as random access. However, such detection performance increases interference between signals while a characteristic of the signal structure is deteriorated according to an influence of a channel.
The following Equation 1 indicates a signal at a receiving end due to an influence of a channel.Rx=Ha×Sa+Hb×Sb  [Equation 1]
If a signal Sa of a transmitting end A and a signal Sb of a transmitting end B are received at a receiving end, the receiving end receives a signal indicated in Equation 1 due to an influence of a channel.
Each sequence and correlation are expressed as illustrated in Equation 2.Da=Ha×Rsa+Hb×Sb×Sa  [Equation 2]
In Equation 2, Rsa denotes autocorrelation of a sequence Sa and indicates a function having a value of 1 at a specific point, that is, a point in which a timing error is 0. Accordingly, if an influence caused by a channel does not exist, then Da=1+delta has a characteristic of a form desired by the sequence. However, if the influence caused by the channel remains, performance is deteriorated by an interference term of Equation 2. To prevent deterioration in performance, signals in a frequency or time region can be discriminated so that an influence of the interference term of Equation 2 can be reduced. To reduce the interference in LTE, a method for varying a root sequence while using other random access sequences and a method for expressing a channel response in different regions using cyclic shift are considered. However, in IEEE 802.16 using a PN sequence, interference in a time region is not reduced and only signals can be discriminated by cross correlation by varying a root index.